The invention relates to a method for producing a biaxially textured metal substrate having a metal surface, which is modified for the production of a high-temperature superconductor (HTS) coating arrangement, wherein the metal surface is modified in order to deposit a layer of the HTS coating arrangement such as a buffer layer, another intermediate layer and/or an HTS layer epitaxially thereon. The invention also relates to a method for producing an HTS coating arrangement in which a buffer layer and/or another intermediate layer and/or a high-temperature superconductor layer are deposited epitaxially on the metal substrate.
In the production of high-temperature superconductor (HTS) coating arrangements, one or more buffer layers are deposited mostly epitaxially on a biaxially textured metal substrate, and a high-temperature superconductor (HTS) layer is deposited on the buffer layer(s). In this case, the buffer layer particularly serves as a diffusion barrier preventing a diffusion of metal atoms or impurities from the metal substrate into the HTS layer. Various deposition processes are known for this purpose, e.g., a vapor deposition of the components forming the buffer layer and/or the HTS layer, e.g., as MOCVD (metal-organic chemical vapor deposition), a chemical solution deposition in which the buffer and/or FITS coating materials are deposited using precursors, or physical processes, such as pulsed laser deposition (PLD), thermal co-evaporation (TCE), etc. Particularly in the chemical solution deposition, a texture transfer from the metal substrate to the HTS layer is required to enable a textured HTS layer to be produced. Correspondingly, if a buffer layer is used, a texture transfer from the metal substrate to the buffer layer and from this to the HTS layer is required.
The metal substrate is important for the properties of the HTS layer in a number of respects, particularly also as regards the achievement of high critical current densities and possibly also the sharpness of the superconducting transfer (ΔTc). The demands on the metal substrate are diverse. For instance, the metal substrate shall have a chemical purity which is as high as possible, a high degree of texturing and, if possible, a surface roughness which is as low as possible in order to enable the growth of a further layer of the HTS coating arrangement, e.g. a buffer layer or another intermediate layer. A roughness of the metal surface which is as low as possible is mostly desired in order to minimize misorientations in the growing layer of the HTS coating arrangement.
Decisive for the quality of the layers deposited on the metal substrate, especially also with regard to the critical current density of the HTS layer, is thus also the surface of the metal substrate, particularly its texture and roughness.
On the one hand, the method according to the invention shall enable the production of preferably optimum growth layers as, for example, buffer layers having a high texturing even on the industrial scale. But the production of metal substrates on the industrial scale is subject to limits, especially with regard to the surface quality which is a pre-requisite for an optimal texture transfer from the metal substrate to the growth layer. Accordingly, in an industrial production process, the biaxially textured metal substrate is mostly produced by a cold-forming process with a degree of deformation of ≧95% and involving a subsequent special annealing treatment. The cold-forming process is mostly performed using rollers. The degree of deformation is one of the decisive factors for the resulting degree of texturing and for the average angle of tilt of the individual crystallites against each other.
On the other hand, the metal substrates which are produced in this way exhibit a relatively high roughness. For various methods for the production of HTS coating arrangements, e.g. in the chemical vapor deposition, this is still acceptable. But especially in the deposition of layers of the HTS coating arrangement by chemical solution deposition, an insufficient texture transfer is observed in such metal substrates. However, increasing the roller nip, which might produce less wear of the rollers and smoother surfaces, would lead to a clearly worse texturing and to an increase in the angle of tilt on the metal substrate. Indeed, it is possible to produce metal substrates having an improved surface quality and enabling an increased texture transfer in the chemical solution deposition under laboratory conditions, for example by using highly polished rollers in the cold-forming process. However, such processes cannot be used on the industrial scale, because the lifetime of the rollers would not be long enough, so that these processes could no longer be carried out on an economical basis. If industrially produced biaxially textured metal substrates are used, only amorphous or only little-oriented growth layers of the respective materials of the HTS coating arrangement (e.g. of the buffer material) are achieved in the chemical solution deposition, despite the good texture of the metal substrate.